Mrs. Madhu is on oral anticoagulant (warfarin) for the prevention of pulmonary embolism. The blood test for monitoring the dose of warfarin is

Appeared in: GMCH - 2015

Explanation

International Normalized Ratio (INR) is the standardized test specifically designed to monitor the effectiveness of warfarin therapy.
Warfarin inhibits the extrinsic coagulation pathway, which is measured by Prothrombin Time (PT). INR is a calculation that standardizes PT results across different laboratories.
For conditions like pulmonary embolism, the therapeutic goal is to maintain the patient's INR between 2.0 and 3.0.
Monitoring INR allows for precise dose adjustments to balance the prevention of clots against the risk of bleeding.

Option A: D-dimer is a diagnostic marker used to rule out the presence of an acute blood clot (thromboembolism). It is not used to monitor the level of anticoagulation.
Option B: Partial Thromboplastin Time (PTT or aPTT) measures the intrinsic coagulation pathway and is the standard test for monitoring unfractionated heparin therapy.
Option D: Clotting Time (CT) is a general, outdated, and non-specific test of the coagulation system. It lacks the sensitivity and precision required for monitoring modern anticoagulant drugs like warfarin.

Clinical Relevance

Nursing practice connection: Knowing Monitoring of Warfarin Therapy helps nurses interpret findings accurately and avoid errors in routine assessment, medication administration, and patient teaching.
Accurate monitoring of warfarin with INR is a critical nursing responsibility to prevent life-threatening complications such as major hemorrhage (if INR is too high) or thrombosis/embolism (if INR is too low).
Patient education is paramount. Nurses must teach patients about the importance of consistent dietary Vitamin K intake, signs of bleeding, and adherence to the INR testing schedule.
What if the patient had severe renal failure? Warfarin is metabolized by the liver and is generally safe in renal failure, so INR would still be the correct test. However, drugs like heparin or some direct oral anticoagulants (DOACs) would require dose adjustments or be contraindicated, highlighting the importance of choosing the right drug and test for the patient's overall condition.

How to Approach the Question

First, identify the key drug in the question stem: 'warfarin'.
Recall the mechanism of action and classification of warfarin. It is an oral anticoagulant that acts as a Vitamin K antagonist.
Associate the drug with its specific laboratory monitoring test. Warfarin's effect on the extrinsic pathway is monitored by Prothrombin Time (PT), which is standardized as the International Normalized Ratio (INR).
Evaluate the options. Eliminate PTT, as it is used for heparin. Eliminate D-dimer, as it is a diagnostic test for clots, not a monitoring tool for therapy. Eliminate Clotting Time as it is a non-specific and outdated test.
Select INR as the only standard, specific, and appropriate test for monitoring warfarin dosage.

Concept Tested & Keywords

Concept Tested: Monitoring of Warfarin Therapy
Stem keywords: oral anticoagulant, warfarin, pulmonary embolism, blood test, monitoring
Lead-in keywords: is
Clinical cues: Patient on warfarin
Clinical cues: Prevention of pulmonary embolism

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Practise the full GMCH - 2015

Attempt every question from this paper in a timed mock, then review the full solution for each one.